It took me, quite literally, three and a half days to find a Tynamo in Chargestone Cave. Then, my Sableye got a crit. Damn!
That reminds me of my one. Finding out that Cryogonal, which I'd encountered and casually KOed on my story run, is a 1% encounter rate outside winter Fortunately it was autumn in-game on the European release, so I didn't have too long to wait.
I remember seeing a Cryogonal back in October last year and I didn't see another Cryogonal since. Then I waited 5 months (now it is February), I took my False Swiper Zangoose to Twist Mountain with me, I wished I would see another Cryogonal, then a wild Pokemon shows up and it's a CRYOGONAL! I used False Swipe on it and it was a pain to catch. I had to use 8 Dusk Balls to catch the darn thing! I was even thinking it was a "Legendary" Cryogonal. I was so relieved when I caught it after waiting 5 months, ironically it was summer when I caught it and they have like, a 1% encounter rate if it's not winter.
It's easier to manually set what Season you want it to be, then you don't have to worry about that low capture rate.
I do that all the time.
In Pokemon Emerald, I tried to catch Registeel for THREE HOURS.
That was so unlucky, must have got a bad number generated.